Dead pixels are hardware not software so the only fix is a new screen or a new device. Take it to an Apple store and they should hand you a new one on the spot. May even work at your carrier store if you bought the phone trough them. If an Apple store is not close by, you can call Apple and make a warranty claim and they will mail you a replacement and you will have to send the old one back.

One dead pixel I don't think is really worth an exchange. I mean if you have other defects and that's an added one I say go for a return, but if that's your main problem with the device I'd say keep it. The pixels on these screens are SO tiny anyway. And a dead pixels are covered in the warranty so if it ever gets too annoying or you get more, you can exchange it anytime during the first year.

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